A polyester composed of an aliphatic diol and an aliphatic dicarboxylic acid as main raw materials, as represented by polybutylene succinate (this polyester will be hereinafter sometimes abbreviated as “polyester”), is processed into products such as agricultural materials, civil engineering materials, vegetation materials, packaging materials, etc. and expected to be utilized over a wide range in view of the facts that it can request the raw materials in materials derived from plant resources; and that it has good physical properties and biodegradability, and the like.
However, a molded article made of the foregoing polyester involved such a problem that when allowed to stand for a certain period of time after molding, cloudiness (hereinafter sometimes referred to as “bleedout” or “whitening phenomenon”) is generated on the surface thereof, whereby the surface gloss vanishes. It may be considered that such a problem is caused due to the fact that at the time of producing a polyester, a cyclic dimer (hereinafter sometimes referred to as “CD”) which is formed simultaneously with the formation of the polyester is deposited on the surface of the molded article after molding and then elapsing a certain period of time. In addition, if the CD content is high, in the case where the polyester is molded into a beverage container or the like and subjected to an elution test by a content liquid, there is also a problem that the elution amount is high so that such a polyester is not suited for containers. Accordingly, a method for eliminating the cyclic dimer from the polyester is studied.
Patent Document 1 describes a method for reducing the CD content by treating a polyester with acetone. In addition, Patent Document 2 describes a method for reducing the CD content by treating a polyester in the form of a powder, a pellet or a molded article with an alcohol or a water/alcohol mixed solution composed of an alcohol as a main component and describes methanol or isopropanol as a specifically applicable alcohol. Furthermore, Patent Document 2 describes a treatment method with a water/alcohol mixed solution composed of water as a main component.